31-Issue 3

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https://diglib.eg.org/handle/10.2312/172

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Automatic Stream Surface Seeding: A Feature Centered Approach
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Edmunds, Matt; Laramee, Robert S.; Malki, Rami; Masters, Ian; Croft, Nick; Chen, Guoning; Zhang, Eugene; S. Bruckner, S. Miksch, and H. Pfister
The ability to capture and visualize information within the flow poses challenges for visualizing 3D flow fields. Stream surfaces are one of many useful integration based techniques for visualizing 3D flow. However seeding integral surfaces can be challenging. Previous research generally focuses on manual placement of stream surfaces. Little attention has been given to the problem of automatic stream surface seeding. This paper introduces a novel automatic stream surface seeding strategy based on vector field clustering. It is important that the user can define and target particular characteristics of the flow. Our framework provides this ability. The user is able to specify different vector clustering parameters enabling a range of abstraction for the density and placement of seeding curves and their associated stream surfaces. We demonstrate the effectiveness of this automatic stream surface approach on a range of flow simulations and incorporate illustrative visualization techniques. Domain expert evaluation of the results provides valuable insight into the users requirements and effectiveness of our approach.
Automating Transfer Function Design with Valley Cell-Based Clustering of 2D Density Plots
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Wang, Yunhai; Zhang, Jian; Lehmann, Dirk J.; Theisel, Holger; Chi, Xuebin; S. Bruckner, S. Miksch, and H. Pfister
Two-dimensional transfer functions are an effective and well-accepted tool in volume classification. The design of them mostly depends on the user's experience and thus remains a challenge. Therefore, we present an approach in this paper to automate the transfer function design based on 2D density plots. By exploiting their smoothness, we adopted the Morse theory to automatically decompose the feature space into a set of valley cells. We design a simplification process based on cell separability to eliminate cells which are mainly caused by noise in the original volume data. Boundary persistence is first introduced to measure the separability between adjacent cells and to suitably merge them. Afterward, a reasonable classification result is achieved where each cell represents a potential feature in the volume data. This classification procedure is automatic and facilitates an arbitrary number and shape of features in the feature space. The opacity of each feature is determined by its persistence and size. To further incorporate the user's prior knowledge, a hierarchical feature representation is created by successive merging of the cells. With this representation, the user is allowed to merge or split features of interest and set opacity and color freely. Experiments on various volumetric data sets demonstrate the effectiveness and usefulness of our approach in transfer function generation.
Biopsy Planner - Visual Analysis for Needle Pathway Planning in Deep Seated Brain Tumor Biopsy
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Herghelegiu, Paul-Corneliu; Manta, Vasile-Ion; Perin, Radu; Bruckner, Stefan; Gröller, Eduard; S. Bruckner, S. Miksch, and H. Pfister
Biopsies involve taking samples from living tissue using a biopsy needle. In current clinical practice they are a first mandatory step before any further medical actions are planned. Performing a biopsy on a deep seated brain tumor requires considerable time for establishing and validating the desired biopsy needle pathway to avoid damage. In this paper, we present a system for the visualization, analysis, and validation of biopsy needle pathways. Our system uses a multi-level approach for identifying stable needle placements which minimize the risk of hitting blood vessels. This is one of the major dangers in this type of intervention. Our approach helps in identifying and visualizing the point on the pathway that is closest to a surrounding blood vessel, requiring a closer inspection by the neurosurgeon. An evaluation by medical experts is performed to demonstrate the utility of our system.
Comparative Evaluation of an Interactive Time-Series Visualization that Combines Quantitative Data with Qualitative Abstractions
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Aigner, Wolfgang; Rind, Alexander; Hoffmann, Stephan; S. Bruckner, S. Miksch, and H. Pfister
In many application areas, analysts have to make sense of large volumes of multivariate time-series data. Explorative analysis of this kind of data is often difficult and overwhelming at the level of raw data. Temporal data abstraction reduces data complexity by deriving qualitative statements that reflect domain-specific key characteristics. Visual representations of abstractions and raw data together with appropriate interaction methods can support analysts in making their data easier to understand. Such a visualization technique that applies smooth semantic zooming has been developed in the context of patient data analysis. However, no empirical evidence on its effectiveness and efficiency is available. In this paper, we aim to fill this gap by reporting on a controlled experiment that compares this technique with another visualization method used in the well-known KNAVE-II framework. Both methods integrate quantitative data with qualitative abstractions whereas the first one uses a composite representation with color-coding to display the qualitative data and spatial position coding for the quantitative data. The second technique uses juxtaposed representations for quantitative and qualitative data with spatial position coding for both. Results show that the test persons using the composite representation were generally faster, particularly for more complex tasks that involve quantitative values as well as qualitative abstractions.
Comparative Visual Analysis of 2D Function Ensembles
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Piringer, Harald; Pajer, Stephan; Berger, Wolfgang; Teichmann, Heike; S. Bruckner, S. Miksch, and H. Pfister
In the development process of powertrain systems, 2D function ensembles frequently occur in the context of multirun simulations. An analysis has many facets, including distributions of extracted features, comparisons between ensemble members and target functions, and details-on-demand. The primary contribution of this paper is a design study of an interactive approach for a comparative visual analysis of 2D function ensembles. The design focuses on a tight integration of domain-oriented and member-oriented visualization techniques, and it seeks to preserve the mental model of 2D functions on multiple levels of detail. In this context, we propose a novel focus+context approach for visualizations relying on data-driven placement which is based on labeling. We also extend work on feature-preserving downsampling of 2D functions. Our design supports a comparison of 2D functions based on juxtaposition, overlay, and explicit differences. It also enables an analysis in terms of extracted scalar features and 1D aggregations. An evaluation illustrates a workflow in our application context. User feedback indicates a time saving of 70% for common tasks and a qualitative gain for the entire development process.
Computing Voronoi Treemaps: Faster, Simpler, and Resolution-independent
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Nocaj, Arlind; Brandes, Ulrik; S. Bruckner, S. Miksch, and H. Pfister
Voronoi treemaps represent hierarchies as nested polygons. We here show that, contrary to the apparent popular belief, utilization of an algorithm for weighted Voronoi diagrams is not only feasible, but also more efficient than previous low-resolution approximations, even when the latter are implemented on graphics hardware. More precisely, we propose an instantiation of Lloyd's method for centroidal Voronoi diagrams with Aurenhammer's algorithm for power diagrams that yields an algorithm running in O(n log n) rather than Ω(n2) time per iteration, with n the number of sites. We describe its implementation and present evidence that it is faster also in practice.
Conceptualizing Visual Uncertainty in Parallel Coordinates
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Dasgupta, Aritra; Chen, Min; Kosara, Robert; S. Bruckner, S. Miksch, and H. Pfister
Uncertainty is an intrinsic part of any visual representation in visualization, no matter how precise the input data. Existing research on uncertainty in visualization mainly focuses on depicting data-space uncertainty in a visual form. Uncertainty is thus often seen as a problem to deal with, in the data, and something to be avoided if possible. In this paper, we highlight the need for analyzing visual uncertainty in order to design more effective visual representations. We study various forms of uncertainty in the visual representation of parallel coordinates and propose a taxonomy for categorizing them. By building a taxonomy, we aim to identify different sources of uncertainty in the screen space and relate them to different effects of uncertainty upon the user. We examine the literature on parallel coordinates and apply our taxonomy to categorize various techniques for reducing uncertainty. In addition, we consider uncertainty from a different perspective by identifying cases where increasing certain forms of uncertainty may even be useful, with respect to task, data type and analysis scenario. This work suggests that uncertainty is a feature that can be both useful and problematic in visualization, and it is beneficial to augment an information visualization pipeline with a facility for visual uncertainty analysis.
ConnectedCharts: Explicit Visualization of Relationships between Data Graphics
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Viau, Christophe; McGuffin, Michael; S. Bruckner, S. Miksch, and H. Pfister
Multidimensional multivariate data can be visualized using many different well-known charts, such as bar charts, stacked bar charts, grouped bar charts, scatterplots, or pivot tables, or also using more advanced highdimensional techniques such as scatterplot matrices (SPLOMs) or parallel coordinate plots (PCPs). These many techniques have different advantages, and users may wish to use several charts or data graphics to understand a dataset from different perspectives. We present ConnectedCharts, a technique for displaying relationships between multiple charts. ConnectedCharts allow for hybrid combinations of bar charts, scatterplots, and parallel coordinates, with curves drawn to show the conceptual links between charts. The charts can be thought of as coordinated views, where linking is achieved not only through interactive brushing, but also with explicitly drawn curves that connect corresponding data tuples or axes. We present a formal description of a design space of many simple charts, and also identify different kinds of connections that can be displayed between related charts. Our prototype implementation demonstrates how the connections between multiple charts can make relationships clearer and can serve to document the history of a user's analytical process, leading to potential applications in visual analytics and dashboard design.
COVRA: A Compression-domain Output-sensitive Volume Rendering Architecture Based on a Sparse Representation of Voxel Blocks
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Gobbetti, Enrico; Guitian, José Antonio Iglesias; Marton, Fabio; S. Bruckner, S. Miksch, and H. Pfister
We present a novel multiresolution compression-domain GPU volume rendering architecture designed for inter- active local and networked exploration of rectilinear scalar volumes on commodity platforms. In our approach, the volume is decomposed into a multiresolution hierarchy of bricks. Each brick is further subdivided into smaller blocks, which are compactly described by sparse linear combinations of prototype blocks stored in an overcomplete dictionary. The dictionary is learned, using limited computational and memory resources, by applying the K-SVD algorithm to a re-weighted non-uniformly sampled subset of the input volume, harnessing the recently introduced method of coresets. The result is a scalable high quality coding scheme, which allows very large volumes to be compressed off-line and then decompressed on-demand during real-time GPU-accelerated rendering. Volumetric information can be maintained in compressed format through all the rendering pipeline. In order to efficiently support high quality filtering and shading, a specialized real-time renderer closely coordinates decompression with rendering, combining at each frame images produced by raycasting selectively decompressed portions of the current view- and transfer-function-dependent working set. The quality and performance of our approach is demonstrated on massive static and time-varying datasets.
A Design Study of Direct-Touch Interaction for Exploratory 3D Scientific Visualization
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Klein, Tijmen; Guéniat, Florimond; Pastur, Luc; Vernier, Frédéric; Isenberg, Tobias; S. Bruckner, S. Miksch, and H. Pfister
We present an interaction design study of several non-overlapping direct-touch interaction widgets, postures, and bi-manual techniques to support the needs of scientists who are exploring a dataset. The final interaction design supports navigation/zoom, cutting plane interaction, a drilling exploration, the placement of seed particles in 3D space, and the exploration of temporal data evolution. To ground our design, we conducted a requirements analysis and used a participatory design approach throughout development. We chose simulations in the field of fluid mechanics as our example domain and, in the paper, discuss our choice of techniques, their adaptation to our target domain, and discuss how they facilitate the necessary combination of visualization control and data exploration. We evaluated our resulting interactive data exploration system with seven fluid mechanics experts and report on their qualitative feedback. While we use flow visualization as our application domain, the developed techniques were designed with generalizability in mind and we discuss several implications of our work on further development of direct-touch data exploration techniques for scientific visualization in general.
Document Thumbnails with Variable Text Scaling
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Stoffel, Andreas; Strobelt, Hendrik; Deussen, Oliver; Keim, Daniel A.; S. Bruckner, S. Miksch, and H. Pfister
Document reader applications usually offer an overview of the layout for each page as thumbnail views. Reading the text in these becomes impossible when the font size becomes very small. We improve the readability of these thumbnails using a distortion method, which retains a readable font size of interesting text while shrinking less interesting text further. In contrast to existing approaches, our method preserves the global layout of a page and is able to show context around important terms. We evaluate our technique and show application examples.
Drawing Large Graphs by Low-Rank Stress Majorization
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Khoury, Marc; Hu, Yifan; Krishnan, Shankar; Scheidegger, Carlos; S. Bruckner, S. Miksch, and H. Pfister
Optimizing a stress model is a natural technique for drawing graphs: one seeks an embedding into Rd which best preserves the induced graph metric. Current approaches to solving the stress model for a graph with jVj nodes and jEj edges require the full all-pairs shortest paths (APSP) matrix, which takes O(jVj2 log jEj+jVjjEj) time and O(jVj2) space. We propose a novel algorithm based on a low-rank approximation to the required matrices. The crux of our technique is an observation that it is possible to approximate the full APSP matrix, even when only a small subset of its entries are known. Our algorithm takes time O(kjVj+jVj logjVj+jEj) per iteration with a preprocessing time of O(k3 +k(jEj+jVj logjVj)+k2jVj) and memory usage of O(kjVj), where a user-defined parameter k trades off quality of approximation with running time and space. We give experimental results which show, to the best of our knowledge, the largest (albeit approximate) full stress model based layouts to date.
Employing 2D Projections for Fast Visual Exploration of Large Fiber Tracking Data
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Poco, Jorge; Eler, Danilo M.; Paulovich, Fernando; Minghim, Rosane; S. Bruckner, S. Miksch, and H. Pfister
Fiber tracts detection is an increasingly common technology for diagnosis and also understanding of brain function. Although tools for tracing and presenting brain fibers are advanced, it is still difficult for physicians or students to explore the dataset in 3D due to their intricate topology. In this work we present a visual exploration approach for fiber tracts data aimed at supporting exploration of such data. The work employs a local, precise and fast 2D multidimensional projection technique that allows a large number of fibers to be handled simultaneously and to select groups of bundled fibers for further exploration. In this approach, a DTI feature dataset, including curvature as well as spatial features, is projected on a 2D or 3D view. By handling groups formed in this view, exploration is linked to corresponding brain fibers in object space. The link exists in both directions and fibers selected in object space are also mapped to feature space. Our approach also allows users to modify the projection, controlling and improving, if necessary, the definition of groups of fibers for small and large datasets, due to the local nature of the projection. Compared to other related work, the method presented here is faster for creating visual representations, making it possible to explore complete sets of fibers tracts up to 250K fibers, which was not possible previously. Additionally, the ability to change configuration of the feature space representation adds a high degree of flexibility to the process.
Graph Bundling by Kernel Density Estimation
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Hurter, Christophe; Ersoy, Ozan; Telea, Alex; S. Bruckner, S. Miksch, and H. Pfister
We present a fast and simple method to compute bundled layouts of general graphs. For this, we first transform a given graph drawing into a density map using kernel density estimation. Next, we apply an image sharpening technique which progressively merges local height maxima by moving the convolved graph edges into the height gradient flow. Our technique can be easily and efficiently implemented using standard graphics acceleration techniques and produces graph bundlings of similar appearance and quality to state-of-the-art methods at a fraction of the cost. Additionally, we show how to create bundled layouts constrained by obstacles and use shading to convey information on the bundling quality. We demonstrate our method on several large graphs.
I-SI: Scalable Architecture of Analyzing Latent Topical-Level Information From Social Media Data
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Wang, Xiaoyu; Dou, Wenwen; Ma, Zhiqiang; Villalobos, Jeremy; Chen, Yang; Kraft, Thomas; Ribarsky, William; S. Bruckner, S. Miksch, and H. Pfister
We present a general visual analytics architecture that is designed and implemented to effectively analyze unstructured social media data on a large scale. Pipelined on a high-performance cluster configuration, MPI processing, and interactive visual analytics interfaces, our architecture, I-SI, closely integrates data-driven analytical methods and user-centered visual analytics. It creates a coherent analysis environment for identifying event structures, geographical distributions, and key indicators of emerging events. This environment supports monitoring, analyzing, and responding to latent information extracted from social media. We have applied the I-SI architecture to collect social media data, analyze the data on a large scale and uncover the latent social phenomena. To demonstrate the efficacy and applicability of I-SI, we describe several social media use cases in multiple domains that were evaluated by experts. The use cases demonstrate that I-SI can benefit a range of users by constructing meaningful event structures and identifying precursors to critical events within a rich, evolving set of topics.
Illustrative Membrane Clipping
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Birkeland, Åsmund; Bruckner, Stefan; Brambilla, Andrea; Viola, Ivan; S. Bruckner, S. Miksch, and H. Pfister
Clipping is a fast, common technique for resolving occlusions. It only requires simple interaction, is easily understandable, and thus has been very popular for volume exploration. However, a drawback of clipping is that the technique indiscriminately cuts through features. Illustrators, for example, consider the structures in the vicinity of the cut when visualizing complex spatial data and make sure that smaller structures near the clipping plane are kept in the image and not cut into fragments. In this paper we present a new technique, which combines the simple clipping interaction with automated selective feature preservation using an elastic membrane. In order to prevent cutting objects near the clipping plane, the deformable membrane uses underlying data properties to adjust itself to salient structures. To achieve this behaviour, we translate data attributes into a potential field which acts on the membrane, thus moving the problem of deformation into the soft-body dynamics domain. This allows us to exploit existing GPU-based physics libraries which achieve interactive frame rates. For manual adjustment, the user can insert additional potential fields, as well as pinning the membrane to interesting areas. We demonstrate that our method can act as a flexible and non-invasive replacement of traditional clipping planes.
Importance Driven Automatic Color Design for Direct Volume Rendering
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Wang, Lei; Kaufman, Arie; S. Bruckner, S. Miksch, and H. Pfister
This paper introduces an automatic color design method that is driven by an importance function of the objects within a volumetric dataset. Our method allows the user to intuitively modify the object classification and the importance distribution function in the 2D rendered image. It automatically computes the transfer function, especially the color distribution, to convey the importance of the objects. In our approach, the importance of an object is represented as the attentiveness of a color. In addition, we preserve the color harmony in the rendered image in order to provide a visually pleasing result. In this paper, we propose a set of computational measurements to compute the color attentiveness and color harmony. Our color assignment algorithm supports arbitrary-dimensional transfer functions and obtains interactive frame rates. Our method involves three color spaces, namely Coloroid system, CIE LChuv, and Adobe RGB color space. It calculates the color attentiveness in CIE LChuv space, and the color harmony in Coloroid system. It, then, assigns the transfer function in a dual space of Adobe RGB space and renders the resulting image in Adobe RGB space. We conducted a detailed user study, which proves that our method successfully conveys the importance distributions. Our contribution in this paper is not only our importance driven approach, but also our computational measurements and our color assignment algorithm.
Interactive Rendering of Materials and Biological Structures on Atomic and Nanoscopic Scale
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Lindow, Norbert; Baum, Daniel; Hege, Hans-Christian; S. Bruckner, S. Miksch, and H. Pfister
The properties of both inorganic and organic materials and the function of biological structures can often only be understood by analyzing them simultaneously on atomic and nanoscopic, if not mesoscopic, scale. Here, the problem arises to render millions to billions of atoms. We propose a method by which it is possible to interactively visualize atomic data, bridging five orders of magnitude in length scale. For this, we propose a simple yet efficient GPU rendering method that enables interactive visualization of biological structures consisting of up to several billions of atoms. To be able to load all atomic data onto the GPU, we exploit the fact that biological structures often consist of recurring molecular substructures. We also exploit that these objects typically are rendered opaquely, so that only a fraction of the atoms is visible. The method is demonstrated on both biological structures as well as atom probe tomography data of an inorganic specimen. We conclude with a discussion about when during ascension from atomic to mesoscopic scale level-of-detail representations become necessary.
Interactive Visualization of Generalized Virtual 3D City Models using Level-of-Abstraction Transitions
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Semmo, Amir; Trapp, Matthias; Kyprianidis, Jan Eric; Döllner, Jürgen; S. Bruckner, S. Miksch, and H. Pfister
Virtual 3D city models play an important role in the communication of complex geospatial information in a growing number of applications, such as urban planning, navigation, tourist information, and disaster management. In general, homogeneous graphic styles are used for visualization. For instance, photorealism is suitable for detailed presentations, and non-photorealism or abstract stylization is used to facilitate guidance of a viewer's gaze to prioritized information. However, to adapt visualization to different contexts and contents and to support saliencyguided visualization based on user interaction or dynamically changing thematic information, a combination of different graphic styles is necessary. Design and implementation of such combined graphic styles pose a number of challenges, specifically from the perspective of real-time 3D visualization. In this paper, the authors present a concept and an implementation of a system that enables different presentation styles, their seamless integration within a single view, and parametrized transitions between them, which are defined according to tasks, camera view, and image resolution. The paper outlines potential usage scenarios and application fields together with a performance evaluation of the implementation.
Interface Exchange as an Indicator for Eddy Heat Transport
(The Eurographics Association and Blackwell Publishing Ltd., 2012) Williams, Sean; Petersen, Mark; Hecht, Matthew; Maltrud, Mathew; Patchett, John; Ahrens, James; Hamann, Bernd; S. Bruckner, S. Miksch, and H. Pfister
The ocean contains many large-scale, long-lived vortices, called mesoscale eddies, that are believed to have a role in the transport and redistribution of salt, heat, and nutrients throughout the ocean. Determining this role, however, has proven to be a challenge, since the mechanics of eddies are only partly understood; a standard definition for these ocean eddies does not exist and, therefore, scientifically meaningful, robust methods for eddy extraction, characterization, tracking and visualization remain a challenge. To shed light on the nature and potential roles of eddies, we extend our previous work on eddy identification and tracking to construct a new metric to characterize the transfer of water into and out of eddies across their boundary, and produce several visualizations of this new metric to provide clues about the role eddies play in the global ocean.

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